1. Field of the Invention
The present invention relates to a fusing device for use in equipment such as copying machines, facsimiles, printers and the like making use of electrophotographic processing.
2. Description of the Related Art
In copying machines utilizing electrophotographic processing, unfused toner images formed on recording sheets have to be fused to change them into permanent images and a heat-fusing method for fusion-bonding the toner image on a recording sheet by heat-fusing toner powder is widely in use now.
As a heat-fusing device, there is a known device including a heating roll having a heater in a cylindrical core bar and a heat-resistant plastic layer formed on the outer peripheral surface of the core bar, and a pressure roll which is forced to contact the heating roll anti has a heat-resistant elastic layer formed on the outer peripheral surface of a cylindrical core bar. The device of the aforesaid heating roll type is used for fusing by passing a recording sheet carrying an unfused toner image through the nip portion between both rolls.
However, such a fusing device of the heating roll type requires the heating roll and when the heating roll is manufactured, expensive heat-resistant elastic material such as silicone rubber, fluororubber or the like is used to form the heat-resistant elastic layer. The problem is that equipment cost is relatively high. Moreover, the contact (nip portion) between the heat and pressure rolls ought to be at least 4 to 10 mm wide to secure predetermined fusing conditions. Consequently, it is necessary to set the roll diameter on the large size or provide a loading mechanism for applying a heavy load, for example, and the heat and pressure rolls are hardly made compact; this naturally results in large-sized and complicated devices.
In order to solve the problems peculiar to the aforesaid heating roll type fusing device, there has been proposed a fusing device to which, in place of a pressure roll, a heat-resistant solid elastic member is applied, the elastic member semicircular in cross section being forced to contact a heating roll (Japanese Utility Model Unexamined Publication No. Sho. 63-62861) . This fusing device is advantageous in that a reduction in not only manufacturing cost but also equipment size is feasible because the pressure roll can be dispensed with.
Notwithstanding, even the fusing device thus proposed still leaves room for improvement in order to deal with various demands growing every year for a reduction in equipment size and cost, and for an increase in performance and so on.